封面.jpg
load方法：
当一个类或者该类的分类被加入Objective-C运行时的时候被调用。load method调用的时机是非常早的，所以你不应该在该方法中去引用其他你自定义的对象，因为你没办法去判断该对象是否已经进行 load method。当然，你可以使用该类所依赖的frameworks，比如Foundation，在你调用load method方法时这些框架已经确保被完全加载成功。当然你的父类也会在这前加载成功。
load调用时机：
1.类本身的load method在所有父类的load method调用后调用。
2.分类的load method在父类的load method调用前调用。
3.类本身load method在分类的load method前调用。
我们现在建立4个文件,在每个文件里面添加load method，并在load method里面打印一句话：
1.png
不做任何操作直接
bulid，我们来看打印台的log：
2.png
这样的结果就证明了我们前面的结论。
源码证明调用时机：
你可以点击
这里下载runtime源码。这里下载的是objc4-706.tar.gz版本。
dyld 是the dynamic link editor的缩写，动态链接器.主要的任务是为了生成可执行文件。
更多了解可以点击这里
1.引导程序初始化：
/***********************************************************************
* _objc_init
* Bootstrap initialization. Registers our image notifier with dyld.
* Called by libSystem BEFORE library initialization time
**********************************************************************/
void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
lock_init();
exception_init();
_dyld_objc_notify_register(&map_2_images, load_images, unmap_image);
}
2.获取在类的列表和分类的列表，如果没有则return;如果获取到则上锁调用prepare_load_methods
void load_images(const char *path __unused, const struct mach_header *mh)
{
if (!hasLoadMethods((const headerType *)mh)) return;
recursive_mutex_locker_t lock(loadMethodLock);
// Discover load methods
{
rwlock_writer_t lock2(runtimeLock);
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
call_load_methods();
}
3.prepare_load_methods方法中你可以看到先遍历了类的列表而不是分类的，所以类的load method会先调用。schedule_class_load方法中你可以看到如果父类有load method，会递归的去遍历，add_class_to_loadable_list然后加入到待加载列表。所以父类的load method方法会比子类先调用。而_getObjc2NonlazyCategoryList直接遍历不做任何操作所以子类的在父类的前面。
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertWriting();
classref_t *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
for (i = 0; i < count; i++) {
schedule_class_load(remapClass(classlist[i]));
}
category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
for (i = 0; i < count; i++) {
category_t *cat = categorylist[i];
Class cls = remapClass(cat->cls);
if (!cls) continue; // category for ignored weak-linked class
realizeClass(cls);
assert(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
//递归加入到到待加载列表
static void schedule_class_load(Class cls)
{
if (cls->info & CLS_LOADED) return;
if (cls->superclass) schedule_class_load(cls->superclass);
add_class_to_loadable_list(cls);
cls->info |= CLS_LOADED;
}
Initialize方法
runtime会发送initialize message当初始化一个类。父类会比子类先接受到这个消息。这个操作是线程安全的，这就是说，当你在某一个线程A里面初始化这个类，这个线程A会去发送Initialize message给这个类对象。如果其他线程B这个时候要发送其他的消息给这个类，线程B会阻塞直到线程A处理Initialize消息完成。
Initialize调用时机：
1.父类会比子类先接受到这个消息。
2.如果子类没有实现initialize mothod,而父类实现了。那么父类的Initialize mothod将会调用多次。
我们现在在'Person'和'Student'文件里面重写initialize method,让后在'viewDidLoad method'添加下面的代码：
- (void)viewDidLoad {
[super viewDidLoad];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"%@ %s 当前的线程：%@", [self class], __FUNCTION__,[NSThread currentThread]);
Student *Student1 = [[Student alloc] init];
Student *Student2 = [[Student alloc] init];
});
}
bulid项目，我们来看打印台的log:
3.png
你会发现：
1.我们做了两次初始化但第二次初始化
Student并没有触动initialize method。2.父类的
initialize method方法比子类的先调用。3.
Student初始化的线程和initialize method线程是相同的。
现在我们注释掉子类的initialize method，再次bulid：
4.png
这个时候
Person父类调用了两次。
源码证明调用时机:
void _class_initialize(Class cls)
{
assert(!cls->isMetaClass());
Class supercls;
bool reallyInitialize = NO;
// Make sure super is done initializing BEFORE beginning to initialize cls.
// See note about deadlock above.
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
// Try to atomically set CLS_INITIALIZING.
{
monitor_locker_t lock(classInitLock);
if (!cls->isInitialized() && !cls->isInitializing()) {
cls->setInitializing();
reallyInitialize = YES;
}
}
if (reallyInitialize) {
// We successfully set the CLS_INITIALIZING bit. Initialize the class.
// Record that we're initializing this class so we can message it.
_setThisThreadIsInitializingClass(cls);
// Send the +initialize message.
// Note that +initialize is sent to the superclass (again) if
// this class doesn't implement +initialize. 2157218
if (PrintInitializing) {
_objc_inform("INITIALIZE: calling +[%s initialize]",
cls->nameForLogging());
}
// Exceptions: A +initialize call that throws an exception
// is deemed to be a complete and successful +initialize.
@try {
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: finished +[%s initialize]",
cls->nameForLogging());
}
}
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: +[%s initialize] threw an exception",
cls->nameForLogging());
}
@throw;
}
@finally {
// Done initializing.
// If the superclass is also done initializing, then update
// the info bits and notify waiting threads.
// If not, update them later. (This can happen if this +initialize
// was itself triggered from inside a superclass +initialize.)
monitor_locker_t lock(classInitLock);
if (!supercls || supercls->isInitialized()) {
_finishInitializing(cls, supercls);
} else {
_finishInitializingAfter(cls, supercls);
}
}
return;
}
else if (cls->isInitializing()) {
// We couldn't set INITIALIZING because INITIALIZING was already set.
// If this thread set it earlier, continue normally.
// If some other thread set it, block until initialize is done.
// It's ok if INITIALIZING changes to INITIALIZED while we're here,
// because we safely check for INITIALIZED inside the lock
// before blocking.
if (_thisThreadIsInitializingClass(cls)) {
return;
} else {
waitForInitializeToComplete(cls);
return;
}
}
else if (cls->isInitialized()) {
// Set CLS_INITIALIZING failed because someone else already
// initialized the class. Continue normally.
// NOTE this check must come AFTER the ISINITIALIZING case.
// Otherwise: Another thread is initializing this class. ISINITIALIZED
// is false. Skip this clause. Then the other thread finishes
// initialization and sets INITIALIZING=no and INITIALIZED=yes.
// Skip the ISINITIALIZING clause. Die horribly.
return;
}
else {
// We shouldn't be here.
_objc_fatal("thread-safe class init in objc runtime is buggy!");
}
}
void _class_initialize(Class cls) 方法里面下面这段代码就是为了递归保证先执行父类的class_initialize method。
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
下面的代码就是为了确认当本类没有实现这个class_initialize method，会再次调用父类的
if (PrintInitializing) {
_objc_inform("INITIALIZE: calling +[%s initialize]",
cls->nameForLogging());
}
